CONCRETE PIPELINES BOOST CAPACITY IN TSHWANE’S PIENAARS RIVER OUTFALL SEWER SYSTEM:

Johann Wessels, Deputy Manager: Replacement and Upgrading of the Tshwane Metropolitan Council’s Water and Sanitation Division, says the system will relieve the old pipeline which drains the rapid developing Kungwini-west area and residential areas such as Waltloo, Nelmapius, Wilgers, Mamelodi and Silver Lakes.

“The new project consists  of two pipeline sections covering a combined distance of some 3.4km,  a 770m long tunnel and two bridges, with a combined length of over 100 meters, spanning the Pienaars River.  Initially the new pipeline sections will be linked through an existing line in Moretele Park. However, this line has also been earmarked for an upgrade in a second phase operation later this year and will entail the laying of 1.7km of HDPE lined concrete piping with an internal diameter of 1 800mm.

“The old pipeline will continue to be used as an emergency bypass system for a substantial number of years. Both the old and new line will drain into the Baviaanspoort Sewer Works on the northern side of the Magaliesberg range, but unlike the old line, which follows the Pienaars River through the Baviaanspoort kloof, the new line takes a more direct route to the works via a tunnel through the mountain,” says Wessels.

Both phases form part of Tshwane’s master plan as compiled by GLS consultants and are expected to serve the area for the next 30 years on a demographic basis of 25 units per hectare.

Situated in Mamelodi, the upper section of Phase 1 covers a distance of 2.9km and comprises HDPE-lined concrete pipe with an internal diameter of 1 500mm. The lower section which links into the tunnel on the south side of the Magaliesberg, comprises 550m HDPE-lined concrete pipe with an internal diameter of 1 800mm. The piping for both sections was supplied by Southern Pipeline Contractors.

The tunnel section which is arch-shaped and 770m long, was constructed in a drill and blast operation through rock with compression strengths of up to 200MPa. The tunnel will initially accommodate the flow from only one 1 800mm section of concrete sewer. However, it has treble that capacity, having been designed to serve the total Pienaars River catchment area - which by any standards is huge - for the next 100 years.

Gerrie van Heerden, a director of GVM, the consulting engineers responsible for designing
Phase 1, says that in contrast to the concrete pipes which are being protected with an HDPE lining, the tunnel wall will be lined with dolomitic aggregate concrete to control corrosion.

“This type of lining allows water seepage to occur. Had the tunnel itself been protected with HDPE lining, it could have resulted in the build up of water pressure behind the lining.

“The new sewer has been designed with an optimum slope of 1:400 and it includes two drop structures to eliminate excessive velocities. We were fortunate in that there was sufficient space to build the line with long radius bends which allowed a 45º bend over a distance of 14m. These sections will comprise 10 pipes 1.4m long with 4.5º deflected sockets. Long radius bends ensure good flow rates without the sudden hydraulic losses typically found in manhole bends. They are also less prone to the settlement of material which can cause maintenance problems,” concludes van Heerden.

Eduardo Cerimele, who heads the project’s main contractor consortium, Cerimele-Phenduka JV, said that some of the challenges on this project have included drilling through large quantities of hard rock and dealing with sections of unstable crumbling shale.

“The crumbling shale meant we had to excavate trenches with battered sides seven to eight metres deep. This was one of the main reasons why concrete piping was chosen for this project. With wall thicknesses designed at 10% of internal diameters, concrete pipes are able to withstand much greater loads than other materials materials,” observes Cerimele.